In the industry quality control after the production cycle in the plant is essential; For this reason, aluminum machining inspection is a fundamental process that allows not only the quality of the different production processes to be evaluated (chip removal, welding, surface and thermal treatments, etc.) but also to correct them when structural or machining defects are detected. .
Inspection in aluminum machining: an essential post-production process
In general, during the production cycle, two quality controls are carried out on the raw material: one, when it enters the factory, to rule out any problem in the raw material; the second control occurs after finishing all the production processes in the plant, when random pieces are taken from a lot and inspected in the laboratory or technical office.
The controls are varied and depend on the process chart; In aluminum there is usually an inspection of welding, threading, the roughness of the surfaces (to determine the correct machining by chip removal) and, of course, the adjustment of the final measurements to the dimensions specified in the corresponding plans.
What is achieved with the inspection in aluminum machining? Rule out any problem that may have existed during the production process by determining, through successive tests, which process or processes caused those defects.
For this purpose, a profile projector is used, with which the pieces are examined to detect any deviation from the desired profile. This is especially important when the threads are rolled.
Calibrated gauges are used or, failing that, go-no go gauges. If the thread is large enough —and its profile standardized— it is also possible to measure them with a Vernier caliper.
External and internal measures
It is the most common aluminum machining inspection. The measurements of the pieces are controlled using calipers, both Vernier (caliper) and Palmer (or micrometer). In general, tolerances in machined aluminum parts are expressed in tenths of a millimeter, although they may require greater precision, of the order of hundredths.
This finish is standard and, therefore, the most requested in aluminum machining inspection with respect to surface finish. It consists of finely scratching the surface of the pieces until the desired roughness is achieved; This can range from a rough finish (with ridges separated by up to a few hundredths of a millimeter) to a mirror polished finish (with ridges indistinguishable by sight and touch).
After polishing, the surface of the pieces is usually treated with anodizing, to avoid the formation of a natural oxide layer, which is sometimes undesirable.
The inspection is carried out visually, with digital microscopes or with profile projectors, which allow scientifically determining the distance between crest and crest, to check that the roughness is according to the plan provided.
It is the passivation of the external surface of any piece of aluminium. It is a thin layer of oxide of this material, which may or may not contain pigments to give it color.
It is generated through an electrolysis process, in which the parts to be anodized are immersed in a cleaning tank and then in another with an electrolytic solution containing, optionally, pigments of different colors.
It is a process that protects the piece against corrosion and gives it a more aesthetic finish than natural aluminum oxide. As an example, aluminum parts intended for mountaineering, camping and premium board games are usually anodized.
Inspection of this process is done by visual comparison with colored cards (similar to the inspection done in the industry graphics, with Pantone cards). The homogeneity of the treatment is also verified, and the correct adhesion of the oxide layer using a cloth, to rule out any discoloration.
This treatment consists of attacking the surface of the finished pieces with a jet of fine vitreous particles; when hitting the piece they leave a tiny indentation, which gives the surface a rough matte finish.
The inspection is carried out visually to confirm that no surface has been left untreated. It can be controlled metrologically by means of a digital microscope.
For this purpose, an instrument called a durometer is used and the Rockwell B scale is adopted (in order to more easily compare the result with those of other harder metals, such as steel or titanium).
It should be noted that due to the nature of this inspection, it is considered destructive, although the piece is only indented with the balls or the diamond point.
A separate paragraph deserves welding in aluminum machining inspection: it is a process that, if not carried out properly, can introduce mechanical and thermal defects in the piece, such as porosities, structurally weak points, cracks or residual stresses.
The visual part of the weld inspection is carried out using a Vernier caliper, to measure the dimensions of the seam —although this is usually done purely visually when the inspector is an expert technician— and determine —by uniting this information with the inspection of the appearance of that seam—whether the welding parameters were correct or not.
These are various aluminum machining inspection mechanisms that, by their nature, cannot be done visually: inspection of cracks and internal stresses, ductile to brittle transitions, and defects due to faulty casting or welding.
Detection of metallographic imperfections such as cracks, bubbles, and phase transitions can be achieved with new ultrasound diagnostic instruments, although such instruments may not yet be available to the general public.
The measurement of internal stresses (a common defect in cast or welded aluminum parts) can be carried out using X-ray diffraction techniques or by a more modern technique —also non-destructive— of ultrasound.
Other destructive inspections
There are destructive tests that can provide information on the raw material more than on the finished piece: they are the tensile, compression, bending, torsion and shear tests, carried out in a material testing laboratory and are intended to determine the quality of the batch. of the raw material.
To carry them out, test tubes of standardized dimensions are manufactured, with the same batch of raw material that will be used to manufacture the pieces; These specimens are tested using different techniques (such as Charpy’s pendulum, tensile and compression tests to determine the ductility and toughness of the material, etc.). If they pass all these tests, the raw material is approved to go into production.